This invention relates to chromatographic apparatus in general, and more particularly to connectors for capillary columns used in such apparatus.
Chromatographic apparatus used for both gas and liquid chromatography typically employ capillary columns to provide control passage ways for substances to be analyzed. Areas of analytical application include gas chromatography, liquid microbore chromatography, capillary electrophoresis, and supercritical fluid chromatography. In most analytical applications today, glass, metal or flexible fused silica capillary columns are used, and such columns typically have internal dimensions in the range from about 0.02 mm to 0.60 mm and external diameters in the range from about 0.10 mm to 0.80 mm. Polymeric capillaries of similar size are also used on occasion. Frequently, it is necessary to join two pieces of capillary columns together in order to repair a broken column, to optimize a chemical separation by joining pieces of dissimilar column, to extend the column length by connecting two columns in series, and to add retention gaps or guard columns. In most analytical applications the column ends must also be connected to a sample injector and a detector.
The requirements placed upon a practical capillary connector for general use in chromatography applications are most demanding. The connector must be usable in regular contact with chemically reactive substances and organic solvents. It must remain leak-free when operated at internal pressures from zero (absolute) to several thousand pounds per square inch. It must not leak whether the fluid in the capillary be a liquid, a gas or a supercritical fluid. The fluid seal provided by the connector must be stable over temperature cycles from sub-ambient to several hundred degrees Celsius. The thermal mass must be small and the thermal conductivity high to maintain thermal equilibrium between the column and its immediate surroundings. In order to avoid degrading the separating power of the capillary columns, the connector must also contribute very little extra volume to the sample flow path and must avoid physical features that might contribute to turbulence and mixing in the fluids. It must make only a negligible contribution to the resistance to fluid flow in the column/injector/detector assembly. Finally, as a matter of convenience, the connection should be easily made without special tools and should be reusable.
In the past, many connectors have been devised to address the need for providing a fluid tight coupling between two capillary column ends. Some such connectors employ a ferrule with a longitudinal bore therethrough for inserting the ends of the columns to be coupled together, and a compression fitting for mechanically compressing the ferrule in order to provide a fluid tight seal between the inner bores of each column end. Another connector employs a cylindrical body with a longitudinal throughbore for receiving the ends of the capillary columns to be joined in a butt connection, and a polyimide sealing resin which is applied to the external joint between each column and the adjacent end wall of the connector in order to seal each column to the connector body. This usually requires heat curing of the resin for a minimum period of time, approximately 20 minutes. Still another connector consists of a hollow glass tube having a double conical configuration formed by forming elongated bulbs in an initially cylindrical tube at appropriate intervals (e.g. 1 cm), the resulting connector having a double-tapered internal bore narrowing from the two outer ends to the central portion thereof. In use, each column end is inserted into a different one of the ends of the connector and moved into a press-fit position. Press-fit glass tubes have also been used with a ferrule and compression fitting at each end to seal against the capillary column.
Known connectors suffer from several disadvantages. The ferrule and compression style connectors require several moving parts and must be carefully assembled and reassembled. Many require separate elastomeric seals to ensure fluid-tight connection. When a ferrule is used, a small piece of column must be cut off after the column is inserted into the ferrule to eliminate debris that might have been forced into the bore of the capillary column. Also, all parts must be chemically inert to the substances used in the analytical chromatographic process and must exhibit good temperature stability. This increases the cost of such connectors. The cylindrical body type connector suffers from the disadvantage of requiring a minimum finite curing time for the resin adhesive in order to provide the requisite fluid seal. In addition, care must be taken in applying the polyimide resin to ensure that no gaps or cracks exist in the adhesive, that the adhesive thoroughly covers the joint to be sealed and that the adhesive does not enter into the lumen of the capillary. In addition, this type of connector cannot be readily reused once the seals have ruptured, or one or more of the capillaries have broken or cracked. The drawn glass double conical connector of the pressfit variety has been reported to suffer from inconsistent fluid seal, particularly with modern high temperature fused silica capillary columns. After heating to elevated temperatures, connections have been observed to begin leaking and the fused silica butts can be either pushed further into the seat or removed therefrom. This had not previously been possible because short term heating above about 200.degree. C. caused the lower temperature polyimide coating on the exterior of the fused silica column to stick so firmly to the connector wall that pulling the connection apart with brute force resulted in the breakage of the fused silica at the seal. Thus, the need exists for a relatively simple column connector devoid of the disadvantages of known devices.